Remote plasma sources (RPS) or in-line plasma sources (IPS) have been used for abatement of perfluorocarbons (PFC's) and global warming gases (GWG's). For example, the RPS or IPS may be installed in a foreline of a vacuum system of a substrate processing system between a high vacuum pump, such as a turbo pump, and a backing pump, such as a dry vacuum pump. However, there is currently no method or apparatus to control an operating pressure of the foreline (and hence the RPS or IPS) to optimize abatement of PFCs and/or GWGs. Accordingly, the inventors have provided improved methods and apparatus for treating exhaust gas in a processing system.